Pub. L. 91–152, title II, § 213(b), Dec. 24, 1969, 83 Stat. 389, provided that the rents fixed by public housing agencies not exceed one-fourth of a low-rent housing tenant’s income be effective not later than ninety days after Dec. 24, 1969, and that the requirements not apply in any case in which the Secretary of Housing and Urban Development determined that limiting the rent of any tenant or class of tenants would have resulted in a deduction in the amount of welfare assistance which would otherwise have been provided to the tenant or class of tenants by a public agency.